My caliber of choice is .308, because it's NATO standard and thus common, the most powerful standard caliber, also available domestically as a sporting round, is inherently accurate, and is 30 cal. Also, due to those factors, I chose a 308 as my primary rifle, so my experience and skill is highest with that round.
Which rifle, on the other hand, is dependent on many factors. If I had to fight in a forest, I'd choose an HK-91 due to its ability to withstand dirt and such, and because I have the most experience with it. If I had to fight on my campus, I'd grab an M14. I don't have one, but I have a Garand, which is darn close, so most of my experience would transfer. Reason for M14 is faster charging, lighter weight, solid stock (for butt-stroking), lower profile, and lower center of gravity. Also easier for someone else to pick up and use. And mags would be easier to find at local gun shops.

While it's hard to argue with the .308, I think I would have to go with the AR-15 in .223. The rifle, ammo and mags are the most plentiful out there. As an anti-personnell round the .223 leaves little to be desired. It is also a lot lighter and I could carry significantly more 223 ammo than I could 308. I love my HK-91, but if I had to hump something across the country, the 91 would probably be staying home!

I'd have to say that it would be someting in .30 caliber. Reason being is that I want the longer range that it would give me. The further away from the enemy I can be and still kill him the better. Since the oponents would most likely be packing either something in 5.56mm or 7.62X39mm I could have them within range of me without me being in range of them.
With that said I'd take my M14 (semi-auto) or M1. The sights on these are hard to beat since they can literaly be calibrated to hit at various ranges. But of course no weapon is any better that the operator so weapon famliarity and target practice is a must.

I'd go with the 308 Win because ammo and rifles are abundant and it has plenty of power to get the job done in most situations. I'd prefer the rifle be either a Savage or Remington bolt action, heavy barrel, with a synthetic stock. Add a 4.5-14x40 or 4-16x40 scope with mil-dot reticle in solid mounts and a Harris swiveling bi-pod and I'd be ready to go. As you can tell, I'm one of the "one shot = one kill" kind of people. I'd rather stay hidden and use one well paced shot than to use a high capacity autoloader and tear up the hillside with 5,000 rounds while accomplishing nothing more than ruining a barrel, wasteing ammo, and killing a few trees and bushes.:assult:

I went with .308 too. Obviously, .308 is all over as mentioned above. I would like an AR-15 .223 for urban or forest conditions, but for a flat, boring state like Illinois (sorry), the range of a .308 comes in handy. On the other hand, you said "North America," so that is a lot of different temperatures and condidtions to fight in, so an AK in 7.62X39 might be nice. Tough choice, but I think I would go with an FN-FAL.

I would thing a 308 would be best in forest warfare to be able to penetrate trees. I heard of a guy shooting an elk with a 30-06 and hitting right on target after going through a small tree. Didn't even know he'd hit the tree till he saw it afterwards.

It was hard to vote against the .308 but I live in heavily forested country. I don't get to many shot opportunities past three hundred yards. You can also carry more ammo. But eventually I will have a .308.

Id go with my AR-15 in .223 because this is the one im used to shooting and the mild recoil can keep me shooting accurate in battle while a 308's recoild would probably jar shooting from offhand positions(.308 battle rifles are heavy to stand up and be able to hit target). The AR-15 would be good because of its light weight and mild recoil all the btter for small framed shoters like me who are young still and wouldnt mind helping my dad if he had to make a North American campaign where street shootings would probably most likely and street fights probably wuldnt extend farther than 300 yards and the .223 can easily kill at these ranges with the right shooter.

But it doesnt matter what rifle it only matters that the shooter can hit what he aims at. if he's only good with certain calibers then he should use these calibers.

I like 8x57 Mauser, in spite of the more arced trajectory because there is enough hitting power even at 700 yards to cause a steel plate to bow from the energy, especially that hot Turk nickel jacket shot from a k98. Tactically, too much bullet for an urban assault/defense, but a great show of force when hitting someone behind cover.

I have a special place in my heart for .303 Brit; It doesn't show it's true ability until it gets past 300 yards...I've been able to make chest shots at 700 yards with my No.4 on a steel silhouette target that the local PD put out for counter-sniping exercise...